Fox and Wambach, U.S. Pat. No. 3,953,394 disclose polyester alloys comprising from 1 to 99 parts by weight of poly(ethylene terephthalate) and from 99 to 1 part of poly(1,4-butylene terephthalate). The alloys can be injection molded into parts which do not delaminate when cooled. Fox annd Wambach also describe such alloys reinforced with fillers, including filamentous glass and also flame retardant modifications of such compositions.
The present invention is based on the discovery of an unusual effect of poly(1,4-butylene terephthalate) in 15% glass reinforced poly(ethylene terephthalate)/poly(1,4-butylene terephthalate) polyblends. It has been found that if the poly(ethylene terephthalate) level in 15% glass reinforced blends with poly(1,4-butylene terephthalate) is increased by 10% increments, very little effect on the physical properties of molded parts is evident until the 60-70% poly(ethylene terephthalate)/40-30% poly(1,4-butylene terephthalate) range is reached. In this range, a rapid drop in heat distortion temperature (DTUL-deflection temperature under load, ASTM Test D648) at 264 psi occurs. This rapid drop can, however, be moved into higher poly(ethylene terephthalate)/lower poly(1,4-butylene terephthalate levels by increasing the mold temperature used for making the workpieces. This reflects a strong effect of poly(1,4-butylene terephthalate) on the crystallization of the poly(ethylene terephthalate) in the polyblends, and was apparently not recognized by Fox and Wambach. There is an improved control in warpage at higher poly(ethylene terephthalate) levels and the property profile remains unaffected at these levels. This unusual effect is due to enhanced crystallinity and will be documented hereinafter.
Once the foregoing strong effect on crystallinity had been discovered, numerous experiments were carried out to determine the effectiveness of internal nucleating agents. Fox and Wambach disclose, broadly, that nucleating agents will improve their polyester alloys, especially those having higher ratios of poly(ethylene terephthalate). Many of the nucleating agents suggested were tried, but only one, not specifically mentioned, sodium stearate, has been found to produce good heat distortion values and physical properties coupled with the ability to produce opaque, crystalline specimens in thin, thick and large/flat parts after injection molding under conventional conditions.
In essence, the present invention provides superior molding compositions within the broad disclosure of the above-mentioned Fox and Wambach patent, which is incorporated herein by reference.